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In many ways interviewing is an art. You have one hour (more if you count the cumulative interview time) to determine if the candidate has the desired skills, and, more importantly, if you would enjoy working with this person. That is a lot of ground to cover.
As if finding out all that information isn't a daunting enough task, you also need to make sure that the candidate has a positive experience while visiting your company (after all, people talk and you want them to be saying good things—since this candidate may not be your next hire, but someone he or she meets may be).
Another tip: Avoid questions that solicit suggestions for improvements to your company's own products. I see two big issues here. The first is that the interviewer has much more information about what ideas have already been tried and what the results were, so it can be very easy to criticize or dismiss a candidate's answers without paying attention to her/his demonstration of creativity and problem-solving skills. Second, the candidate may feel uncomfortable providing ideas to the potential employer in this capacity, as s/he is not yet an employee of the company but yet is asked to provide knowledge to further its business. Better to ask questions about fictitious products, or possibly have the interviewee pick a topic of interest.
Always ask the candidate a question that covers a real problem in your company and that YOU DONT KNOW THE ANSWER TO.
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